This invention relates to data communication systems and to systems for synchronizing and supervising apparatus and devices at a plurality of remote locations. More particularly, I disclose a system which employs existing radio broadcast stations to transmit synchronizing and supervisory signals for use by apparatus and telephone communication devices at remote locations for timing purposes and for coordinating their activities with activities at other locations.
In the above identified parent application there is disclosed an improved connumication system for electric utility load management and distribution automation, and the like. That system includes a commercial AM radio broadcast station as the principal means of communicating digital signals to remote locations for the purpose of controlling electrical loads to minimize power demand during critical heavy use periods and to control devices which improve the efficiency of the electrical distribution system. The digital signals are superimposed upon the existing voice transmissions of the broadcast station in a non interfering manner. In the invention disclosed herein that system is extended to include synchronization and supervision of telephone networks, and the like, using what the inventor calls a "TELRAD" modem (acronym for Telephone-Radio). The TELRAD modem includes a radio receiver tuned to the broadcast station, circuits for generating subcarrier signals for transmission of data over the phone lines such as the existing dial-up network, and the corresponding circuits for detecting and demodulating said subcarriers.
A principal distinguishing feature of this system is incorporation of an independent communication media (i.e., the broadcast signal) to a multiplicity of remote locations as well as the existing phone lines and this permits supervision of the primary communication media (the telephone). More specifically, the broadcast signal sets up telephone links by first communicating a precursor "seize telephone line" command, whereupon the remote device prepares to receive an incoming call. When the ring signal commences, it quickly seizes the telephone line and suppresses the ring to establish a communication path without alerting or disturbing people at the remote location. In addition, the broadcast signal continuously synchronizes digital bit streams, message frames, and other activities at the plurality of remote locations and this simplifies equipment design, increases reliability, and reduces cost of TELRAD modems.